1. Field of the Invention
The present invention relates to polymeric blends based on vinyl-aromatic polymers, and more particularly to blends which contain an alkenyl-aromatic resin and are suitable to be transformed into shaped articles by injection molding extrusion or other known processing techniques for thermoplastic polymers.
2. Description of the Prior Art
From British Patent Application No. 2,067,579A are known vinyl-aromatic polymers which possess an improved stress cracking resistance and are particularly suitable to be molded or vacuum shaped.
These vinyl-aromatic polymers modified with rubber are characterized in that they contain from 6 to 12% by weight of an ethylenically unsaturated nitrile, such as acrylonitrile, bound to the two phase system of the polymer, and exhibit the following particular combination of characteristics:
(a) the dispersed elastomeric gel phase insoluble in toluene is not less than 23% by weight; PA0 (b) the swelling index of the elastomeric phase in toluene is greater than 10; PA0 (c) the melt index of the polymer is at least 1.5 g./10 min.; PA0 (d) the bending modulus of the polymer is greater than 15,000 kg/cm.sup.2 ; PA0 (e) the torsional modulus of the polymer is greater than 5,500 kg/cm.sup.2 ; PA0 (f) the notched IZOD impact strength at 23.degree. C. of the polymer is greater than 7 kg.cm/cm. PA0 from 20 to 80% by weight, with respect to the blend, of a modified vinyl-aromatic polymer containing from 5 to 15% by weight of an ethylenically unsaturated nitrile; PA0 from 80 to 20% by weight, with respect to the blend, of at least one resin containing aromatic groups selected from: PA0 1. The heat distortion temperature (HDT) was determined according to the standard ASTM D 648, at 66 psi and at 264 psi. PA0 2. The Vicat softening point procedure A was determined according to the standard ASTM D 1525. PA0 3. The IZOD impact strength was determined at 23.degree. C. according to the standard ASTM D 256, by using test pieces of size 1/2".times.1/8" (1.27 cm.times.0.32 cm). PA0 4. The tests for the resistance against Freon (ESC) were carried out on test pieces subjected to creep in a tensile test, maintaining their central part, for a 40 mm stretch, in contact with liquid Freon 11. For this purpose, a glass container containing Freon 11 was fixed, by means of a rubber gasket, to the lower end of the vertically arranged test piece, in correspondence with its wider stretch. The test piece was subjected to a load of 100 Kg/cm.sup.2 and the time necessary for the rupture of the same was measured.
The methods of measuring the above-mentioned characteristics are described in the Patent Application No. 2 067 579A.
Such modified vinyl-aromatic polymers are particularly suitable to be used in the production both of refrigerator cells and of other articles obtained by thermoforming the extruded sheets. Such polymers can be easily manufactured by processes which are presently in use. Furthermore, they are compatible both with polystyrol or impact resistant polystyrol, as well as with ABS thermopolymers and styrol/acrylonitrile copolymers (SAN).
Nevertheless, these vinyl-aromatic polymers modified with an ethylenically unsaturated nitrile, and also in general all vinyl-aromatic polymers, have the drawback of having a low heat distorsion temperature so that their thermal resistance is not sufficiently satisfactory to meet the necessary requirements in some fields of application, for example in the automotive industry.